The invention pertains to unipolar semiconductor devices especially useful as amplifiers of micro and millimeter wave signals.
Field Effect Transistors are commonly used as amplifiers of high frequency electromagnetic signals, and heretofore have been most commonly used in common source amplifier circuits. Unfortunately, common source amplifiers have the inherent limitation that the drain to ground (output) impedance cannot be made equal to the gate to ground (input) impedance, a severe limitation on power transmission along a gate stripe. An alternate statement of this problem is that in no common source amplifier circuit can the phase velocity of the input signal equal that of the output signal, making phase cancellation between input and output stages unavoidable. Heretofore this problem has been overcome by making such amplifier circuits very small in the direction of wave propagation, thus severely limiting the power handling capacity of such amplifiers. Because common gate amplifier circuits can, theoretically, have impedance matched inputs and outputs, an FET device specially adapted to internally match its input and output impedance could form the basis of an especially effective high frequency amplifier.
Another problem facing designers of high frequency amplifiers is material limitations that restrict high frequency range. To achieve high gain, one often must use plural amplifier stages; however, increasing the number of stages also increases the transit time of charge carriers across the device, limiting the device's response time, i.e., upper frequency range. Moreover, a peculiar property of some semiconductors (e.g., compound semiconductors of III-V material) i.e. whose elements are taken from columns III and V of the Periodic Table is that drift velocity of charge carriers increases with increasing field strength only up to a point, and thereafter decreases with increasing field strength This means that after a certain point, increasing field strength actually decreases device response time, and the device's upper frequency range.
Finally, because these amplifiers, like any semiconductor amplifiers, are regularly fabricated unitarily in monolithic chips, the more the number of fabrication steps, and the delicacy and complexity of each step, the more the likelihood of some fatal error that would render a chip fatally defective.